1. Field of the Invention
The present invention relates generally to a semiconductor device and particularly to a super high-speed semiconductor device having a gate insulator made of a silicon nitride film and a method of fabricating the same.
2. Description of the Related Art
In a semiconductor integrated circuit device such as the CMOS-LSI where super high-speed operation is greatly demanded, the field effect transistor (MOSFET) making up the semiconductor integrated circuit device must have a very short gate length. Thus, a great effort is being made in miniaturizing the MOSFET.
In the miniaturized MOSFET, as the gate length of the MOSFET is decreased, the film thickness of the gate insulator is also restricted due to the scaling law. For example, in a semiconductor device with a gate length below 0.1 μm, the film thickness of the gate insulator must be reduced to a silicon oxide film-equivalent thickness of no more than 2 nm.
Conventionally, a silicon oxide film, which generally has good leakage current characteristics and a low interface state density, is used as the gate insulator. However, in the conventional gate insulator made of a silicon oxide film, the direct tunneling current increases with the reduction of the physical film thickness of the gate insulator. Thus, when the film thickness of the gate insulator is reduced even further than the above value, the gate leakage current due to the tunneling current will be a big problem. For example, when the gate leakage current increases, a substantial leakage current may be generated even when the gate is turned off and the circuit of the semiconductor device may not operate properly, or substantial problems owing to physical laws such as the increase in electricity consumption may arise.
In the conventional art, the silicon oxide film is replaced with a high dielectric constant film, which has a small electric equivalent thickness in spite of a large physical thickness, as the gate insulator.
The gate capacity C can generally be expressed as C=εOεKA/dK, where εO is the permittivity (i.e. the dielectric constant) in a vacuum; εK is the relative dielectric constant of the insulator; A denotes the area of the insulator; and dK denotes the film thickness of the insulator. When a high dielectric constant film, which has a higher relative dielectric constant εn than that of the silicon oxide film, is used as the gate insulator, the film thickness dP of the above high dielectric constant film that realizes the same gate capacity as that of the silicon oxide film can be expressed as dP=εP/εox×dox, where εox and dox represent the relative dielectric constant and the film thickness of the silicon oxide film, respectively. Thus, a gate insulator implementing a high dielectric constant film having electric characteristics that are equivalent to those of a silicon oxide film with a thickness of dox can be realized with a physical film thickness of dP. The relationship between the film thickness of the silicon oxide film dox and the physical thickness of the high dielectric constant film dP having the equivalent electric characteristics can be expressed as dox=εOX/εP×dP, where the ratio of the relative dielectric constant εOX of the silicon oxide film to the relative dielectric constant εP of the insulator is used as the coefficient. The film thickness dOX of the silicon oxide film calculated from the actual film thickness dP of the insulator using the above equation is called the silicon oxide film-equivalent thickness.
For example, a silicon nitride (Si3N4) film, which has twice the relative dielectric constant as that of the silicon oxide film, can be used as the gate insulator of the MOS transistor making up the super high-speed CMOS (Complementary MOS) element that has a gate length below 0.1 μm. The silicon nitride film has better compatibility with the CMOS fabrication process compared to metal oxide insulators such as Al2O3, ZrO2, or Ta2O5 insulators, and can be easily implemented to the gate insulator. Thus, for example, even when the physical film thickness of the gate insulator is more than 2 nm, a silicon oxide film-equivalent thickness of 2 nm or less can be realized, and this enables a faster operation of the semiconductor device as well as the prevention of the gate leakage current due to the tunnel effect.
In a case where the silicon nitride film is directly mounted onto the silicon substrate, nitrogen is condensed on the interface between the silicon substrate and the silicon nitride film, and the carriers transported at a fast rate in the channel region may possibly be scattered. In other words, in the above structure, the mobility of the carriers is decreased due to the nitrogen at the interface, and the threshold characteristics of the semiconductor device are also destabilized due to the carriers being trapped by the dangling bonds in the silicon nitride film.
For example, the formation of the silicon nitride film is usually accomplished using the CVD (chemical vapor deposition) technique; however, the silicon nitride film formed using this technique has many defects and dangling bonds as well as hydrogen atoms that terminate as in the above. Thus, it is difficult to obtain the appropriate film quality of the gate insulator formed in the channel region where the carriers are transported at a fast rate. Also, in a case where thermal processing is performed in a nitrogen atmosphere in order to resolve the above defects, the nitrogen atoms are diffused onto the interface of the silicon substrate thereby causing the dispersion of the carriers and the loss of its mobility. Further, when the dangling bonds trap the carriers, the threshold characteristics of the semiconductor device change.
In response to the above problems, thermal processing of the CVD silicon nitride film in an oxide gas atmosphere can be contemplated so as to avoid the problems arising from the thermal processing in a nitrogen atmosphere; however, in such a case, the oxygen atoms are diffused onto the interface of the silicon substrate thereby lowering the dielectric constant of the silicon nitride film and causing an oxide film to be formed on the interface of the silicon substrate and the gate insulator. The formation of the oxide film cancels out the effects of using a silicon nitride film with a high dielectric constant; thus, such an effect should be avoided by all means.
Alternatively, there is a technique of performing a plasma nitridation process on the surface of the silicon oxide film to form a silicon nitride film; more precisely, a technique of forming a silicon nitric-oxide film has been proposed in the conventional art. For example, this technique is disclosed in: VLSI Symposium 2001, session T7A-4. According to this technique, nitrogen atoms and oxygen atoms are intermingled within the silicon nitric-oxide film, and the segregation of the nitrogen atoms on the interface of the silicon substrate and the silicon nitric-oxide film can be prevented.
FIGS. 1A through 1C illustrate the steps in forming the high dielectric constant gate insulator through plasma nitridation of the above silicon oxide film.
In FIG. 1A, a thermal oxide film 202 with a film thickness of 2 nm, for example, is formed on a silicon substrate 201; in FIG. 1B, the plasma nitridation process is performed on the above thermal oxide film 202 so that it is nitrided by nitrogen radicals N*; and in FIG. 1C, the resulting silicon nitric-oxide film is shown.
As previously mentioned, in the silicon nitric-oxide film formed in the above manner, nitrogen atoms and oxygen atoms are intermingled, and the segregation of the nitrogen atoms on the interface of the silicon substrate can be prevented. Thus, by using the above silicon nitric-oxide film as the gate insulator, the carriers being transported at a fast rate in the channel region can be prevented from being scattered by the nitrogen atoms on the interface of the gate insulator and the silicon substrate.
However, in the above conventional art, the film 203 functioning as the gate insulator is made of a silicon nitric-oxide film (SiON film), which has a smaller dielectric constant compared to the silicon nitride film; therefore a dramatic reduction of the silicon oxide-equivalent thickness cannot be expected from such a structure. In other words, although the relative dielectric constant of the silicon nitric-oxide film is affected by the nitrogen concentration in the film, when the nitridation process is performed on the SiO2 film, the quantityof the nitrogen atoms that may be incorporated into the film is limited since the oxygen atoms already reside in the film.